Devonian Plants from Colombia, with Discussion of Their Geological and Palaeogeographical Context
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Geol. Mag. 137 (3), 2000, pp. 257–268. Printed in the United Kingdom © 2000 Cambridge University Press 257 Devonian plants from Colombia, with discussion of their geological and palaeogeographical context CHRISTOPHER M. BERRY*, EDUARDO MOREL† , JAIRO MOJICA‡ & CARLOS VILLARROEL‡ *Department of Earth Sciences, University of Wales Cardiff, P.O. Box 914, Cardiff CF10 3YE, Wales, UK †Departamento de Paleobotánica, Museo de la Plata, Paseo del Bosque s/m, 1900 La Plata, Argentina ‡Universidad Nacional de Colombia, Departamento de Geociencias, Apartado 14490, Bogotá, Colombia (Received 24 June 1999; accepted 25 January 2000) Abstract – Plant fossils are described from the Cuche Formation, Eastern Cordillera, Colombia in the area of Floresta. Those identified as Colpodexylon cf. deatsii Banks and cf. Archaeopteris sp. suggest an earliest Late Devonian (Frasnian) age for the formation. These or similar taxa are also found in contemporaneous deposits in western Venezuela, and other elements of the Venezuelan flora are found in a geographically intermediate locality. All three Devonian plant localities in the northwest of South America are within the Colombian Eastern Cordillera and its northern extension, the Venezuelan Perijá Range, an area that has been integrated as a part of the so-called ‘Eastern Andean Terrane’ or ‘Central Andean Province’, supposedly accreted to the autochthonous block of the Guyana Shield during the early Jurassic or before. Although both invertebrates and plants from this terrane have strong affinities to North American and European assemblages, and might be interpreted as implying a Laurussian origin for the Eastern Andean Terrane, the evidence is not yet unequivocal, with some authors postulating an in situ development of this province. exposures (for the most part Middle Devonian) are 1. Introduction sparse within this area, and occur generally in massifs Records of vascular plants are rare in Devonian sedi- with Precambrian to ?Early Palaeozoic crystalline ments of South America. The most comprehensive cores. The Floresta Massif is composed of a crys- reports have been from the Middle and Upper talline core unconformably covered by Devonian sedi- Devonian of western Venezuela (Berry, Edwards ments that include the El Tibet, Floresta and Cuche and co-authors) and the Silurian/Lower Devonian formations. The El Tibet Formation has a restricted of Bolivia/Argentina (Morel, Edwards & Iñigez distribution and thickness ranging up to 425 m (M. G. Rodriguez, 1995). This scarcity of fossil evidence has Paz & A. D. Urrutia, unpub. Trabajo de Grado, frustrated attempts at global phytogeography during Departamento de Geociencias, Universidad Nacional these times (Edwards & Berry, 1991). de Colombia, Bogotá, 1996). It is composed mainly of The present report concerns fossil plants from fine- to coarse-grained quartzitic and kaolinitic sand- Devonian strata of the Cuche Formation, outcropping stones, deposited essentially in a continental environ- in the Eastern Cordillera, Colombia around Floresta ment (braided streams) and coastal to subcoastal in the Department of Boyacá (Figs 1, 2), and is com- conditions. On the basis of a spore assemblage, plimentary to work on fossil vertebrates in the same Grösser & Prössel (1994) defined an Emsian age for region (Janvier & Villarroel, 1998). this unit. The field observations and collection of the samples The overlying Floresta Formation (Olsson & Caster, were accomplished by JM and CV, further samples 1939) contains fossil invertebrates (brachiopods, trilo- collected by Professor Medardo Chacón, and study of bites, bryozoans, gastropods, etc.) as described by the fossil plants was undertaken by CMB and EM. Caster (1939), Royo y Gómez (1942), Morales (1965), and fish remains of rhenanid and arthrodire type (Janvier & Villarroel, 1998). The fossil content indi- 2. Geological background and localities cates a Middle Devonian age for the Floresta As shown in the publications of Barrett (1988) and Formation. Forero (1991), known outcrops of sedimentary The Cuche Formation (Botero, 1950) is the Devonian rocks in Colombia are completely restricted youngest of the Palaeozoic sedimentary sequences to a region comprising the Eastern Cordillera, its developed in the Floresta Massif. It represents a northward extension in the Sierra de Perijá and the regressive unit deposited in a transitional environment upper Magdalena Valley. However, the Devonian that includes tidal to supratidal sandstones (Lower Member) and lagoonal siltstones containing a fossil * Author for correspondence: [email protected] assemblage of vertebrates, invertebrates and plants http://journals.cambridge.org Downloaded: 24 Feb 2014 IP address: 131.251.254.13 258 C. M. BERRY AND OTHERS Figure 2. Location map of the two fossil localities men- tioned in this report: (1) Puerto Arepas; (4) Quebrada Potrero Rincón. Modified after Mojica & Villarroel, 1984. Figure 1. Devonian outcrops yielding fossil plants in Colombia and northwestern Venezuela (stars) in relation to known in the adjoining Santander Massif, studied in selected terranes. AUT= Autochthonous Terrane (Guyana detail by Rabe (1977). Shield), EAT = Eastern Andean Terrane (including Sierra de Mojica & Villarroel (1984) provided a list of plant Perijá and Eastern Cordillera). Modified from Restrepo & Toussaint, 1988. fossils from the Cuche Formation, including the gen- era Psilophyton Dawson emend. Banks, Leclercq & Hueber and Ginkgophyton sensu Beck. Other fossils include the ostracod Welleria Ulrick & Bassler and which reflect the transition from typical shallow fish remains. On this basis Mojica & Villarroel (1984) marine conditions to a nearshore terrestrial domain. assigned an age from Middle Devonian to Early This unit is made up of beds of sandstones and silt- Carboniferous to the Cuche Formation. However, stones, which have a concordant and transitional rela- recent study of the vertebrate fauna has allowed tionship with the underlying Floresta Formation. It restriction of the time of deposition to the Late consists of a Lower Member (140 m) of sandstones Devonian Epoch, specifically to the Frasnian (Janvier and an Upper Member (250 m) in which there are & Villarroel, 1998). thick beds of red, violet and ochre sandstones and silt- The plant fossils studied in this paper include the stones. The Lower Member contains many small, same specimens reported by Mojica & Villarroel unidentifiable fragmentary plant remains, but the (1984) as well as others, collected later by JM and CV. Upper Member contains several beds with well-pre- The fossils described here were collected near served fossil plants. The depositional environment has Floresta, in the Eastern Cordillera (Fig. 1). Most been interpreted as shallow water, of varying energy, derive from the section through the upper part of the with occasional subaerial exposure (Mojica & Floresta Formation and the Cuche Formation in the Villarroel, 1984). The Cuche Formation is overlain dis- upper part of Quebrada Potrero Rincón (Fig. 2, local- cordantly by a thick sequence of red beds (conglomer- ity 4) described by Mojica & Villarroel (1984, their figs ates, sandstones and siltstones) resembling the Upper 3, 4). A complimentary stratigraphic section was pro- Jurassic Girón Formation (Hettner, 1892). After the vided by Janvier & Villarroel (1998, their fig. 2). The deposition of the Cuche Formation and before the best preserved and most informative plant specimens accumulation of the Girón-like red beds, the Floresta are from assemblage III (Mojica & Villarroel, 1984, Massif was uplifted leading to no deposition or ero- their fig. 3), derived from a locality made up of sandy sion of the Carboniferous and Permian sequences shales of a yellow/brown colour which also contain http://journals.cambridge.org Downloaded: 24 Feb 2014 IP address: 131.251.254.13 Devonian plants from Colombia 259 fish scales and bones (Janvier & Villarroel, 1998, separated by a distance of approximately 1.6 mm. Leaf locality PR-1). Plant fossils include the specimen of bases alternate in successive gyres. Leaf bases are also Colpodexylon, the unnamed trimerophyte derivative, visible on the left-hand margin of the stem where they cf. Archaeopteris and some of the leafy fossils. The are thickened compared to the proximal leaf lamina, horizon is in the middle of the Upper Member of the appearing decurrent in compression beside the stem. At Cuche Formation. the widest part of the remaining stem there are a maxi- A single large leaf, Mch-019-e, was recovered from mum of six leaf bases visible in any gyre, implying a min- the same section in Quebrada Potrero Rincón, imum of 12 in any complete rotation. towards the bottom of the Lower Member, where rain drop impressions (‘gotas de lluvia’) are recorded just Leaf morphology. A single basal portion of a leaf below thick beds of reddish sandstones (Mojica & going down into the matrix below the stem has a width Villarroel, 1984, fig. 3). of 1.0 mm. Other examples uncovered compressed Other large leaves have been found in the site named beside the stem are much thinner (0.4–0.6 mm), this ‘Puerto Arepas’ on the south side of the road from being due to compaction and folding of the lamina. Duitama to Sogamoso, just below an angular un- Four leaves have been uncovered and are com- conformity with the conglomerates of the Early paratively well preserved. Their measurements are Cretaceous Tibasosa Formation. This locality is recorded in Table 1. Lengths of leaves and segments marked as 1 on Figure 2. These fossils are